Poly vinylidene fluoride (PVDF) is a polymer known for its excellent properties, particularly mechanical firmness and firmness in respect to chemical agents. However, it has the disadvantage of lacking suppleness and hence, this disadvantage limits its use in applications where this property is required, such as in tubular materials that must be rolled up.
To overcome the disadvantage, it has been proposed to copolymerize the VF.sub.2 with C.sub.2 F.sub.3 Cl, as, for example, has been described in Japanese Patent Application No. 51-149392/76. According to this patent, the copolymerization is carried out by uniformly adding the C.sub.2 F.sub.3 Cl to the reaction medium containing the entire amount of the VF.sub.2. In this type of copolymerization the speed at which the C.sub.2 F.sub.3 Cl is introduced to the VF.sub.2, taking into consideration the differences of speed of reaction between the two monomers, that of the VF.sub.2 being slower than that of the C.sub.2 F.sub.3 Cl, is adjusted so as to obtain a homogeneous copolymer of the type (--CF.sub.2 --CH.sub.2 --CF.sub.2 --CH.sub.2 --CF.sub.2 --CFCl--).sub.n.
The homogeneous copolymers thus obtained have a high degree of flexibility, however, their melting point decreases as the number of --CF.sub.2 --CFCl groups increases in the macromolecular chain. In addition, when 15% by weight of C.sub.2 F.sub.3 Cl in the copolymer is exceeded, elastomers are formed, which in homogeneous structure, are known as products which have weak resistance to rupture and to discharge threshold, but of high tensile strength.